This is only based on observation from people posting on the forum. It seems that once people got the ninja from these RNG events right once the events are released, many will try right after but won't get it. It could be just the frequency of players trying just increased because they think they can get it as much as the one who got the ninja.

Also, get rid of the mindset that just because you're a f2p player you won't ever get a ninja from events and the only way is by luck. You can still get ninjas that you can get from events but it just takes more time in comparison to those who paid to get them since you just need to farm coupons. If you played since July, you should have gotten about almost 35-40 free frags of masked or jonin by now.